The invention relates to packet-switched transmission of calls and particularly to packet-switched transmission of calls on circuits between a telecommunication network and base stations from the base stations toward the network in a mobile telephone system in which a call is transferred as data packets at even intervals and in which a carrier wave of the radio path is employed by using time division.
In most digital mobile telephone systems the carrier wave of the radio path is divided between several users by using, for example, a TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) method. The common feature of the methods is that a different channel is allocated for each call, and that the call is divided into call packets of a certain length and the packets are transferred on different channels in frames transmitted in succession on the carrier wave of the radio path. A frame comprises several channels, and the time needed to transmit a frame is constant. In the TDMA method, where a frame consists of time channels that are successive with respect to time, packets associated with one and the same call are received at the base station at an interval of the duration of the transmission of a frame.
On the circuits of a network infrastructure, calls are usually transmitted using a circuit-switched system, whereby a different transmission channel, or even a different channel in each direction, is allocated for signalling and for each call. In a circuit-switched system a channel remains allocated even if nothing is transmitted, whereby the efficiency on a 64 kbit/s transmission connection is poor. Further, in a circuit-switched system the network capacity will be abruptly finished as the load increases, whereas in a packet-switched network an increase in the load appears as a reduction in the transmission rate and deterioration of the quality of the connection.
In the known packet-switched networks, packets are usually forwarded in the order of arrival and/or generation. The packets may then have to queue in a network node, if the transmission of a preceding packet is still in progress. Unanticipated queuing of a varying length causes variation in the packet transmission times and prolongs the maximum transmission delay. The variation in the packet transmission times is particularly harmful where speech transmission is concerned, since the quality of speech is impaired by the variation in the transmission times. In addition, in speech transmission it is important that the maximum delays are short.
If a packet-switched network node receives packets from a lower-rate transmission channel and forwards them to a higher-rate transmission channel, some of the advantages brought about by the, higher-rate transmission channel may be lost in waiting for the termination of the transmission of a preceding packet. However, it should be possible to forward particularly speech call packets immediately after they have been received, so that the packet transmission times would not vary because of variation in the waiting times and so that the maximum delay caused by the waiting times would be as short as possible and high-quality speech transmission could thus be guaranteed.
The above problems, i.e. the variation in the transmission delays of the packets caused by the waiting and the increase in the length of the maximum delays, can be overcome, for example, by transferring the packets by emulating circuit-switching, i.e. by allocating a separate channel or channels for them. The problems in this solution are the same as in a circuit-switched solution, i.e. capacity is allocated although it may not be needed, and as the load increases the capacity will be abruptly finished.
Another way of levelling the transmission delays of the packets in the packet-switched network is to buffer a sufficient number of packets at the receiving end, so that any variation in the transmission delays can be levelled. The problem is that real-time transmission of speech does not allow large buffering: in a public safety and security call, for example, the delay must be less than 400 milliseconds. In the Internet the speech-transmitting software uses large buffering, for a delay of a couple of seconds is irrelevant there in speech transmission. Since speech must be put through as quickly as possible in a system transmitting real-time speech, the buffer must be very short, and so buffering cannot be relied on to level the transmission delays. Further, the buffering increases the maximum delay of the packet transmission time.
The object of the present invention is to provide a method and a network node by which the maximum delay of the call packets and the variation in the transmission times can be reduced. The object is achieved by a packet-switched data transmission method of the invention, in which call packets are separated from control packets containing signalling information on the data transmission network and which is characterized by maintaining information on the arrival rate of the call packets; calculating the duration of the transmission of the control packet that is to be sent next; defining the interval to the reception of the next call packet; comparing the duration with the interval, and if the duration is shorter than or equal to the interval, transmitting the control packet, and if the duration is longer than the interval, putting the control packet in a transmission queue, waiting until said call packet has been received, and subsequently forwarding the call packet substantially immediately.
Substantially immediately means that the packet is forwarded immediately after the measures necessary in receiving a packet have been taken and the packet is ready for forwarding. The measures include, for example, detecting whether the packet is a call packet, possibly checking the address, and receiving the packet in full before forwarding it if the packet is received from a lower-rate channel than it is transmitted to.
The invention also relates to a network node that can be utilized in the method of the invention. The network node of the packet-switched transmission network of the invention, to which there leads at least one incoming channel and from which there leads at least one outgoing channel and which comprises reception means for receiving packets, transmission means for transmitting packets toward a destination address, and a buffer for buffering the received packets, the reception means being arranged to separate call packets from control packets that contain signalling information on the data transmission network, is characterized by the node further comprising memory means for storing the information indicating the arrival rate of the call packets, a time counter for defining the interval to the reception of the next call packet, calculating means for calculating the duration of the transmission of the control packet to be sent next, comparison means for comparing the interval and the duration, and a transmission order controller for putting the control packets in a transmission queue in the buffer and for transmitting said next call packet next, if the duration is longer than the interval.
The invention is based on the idea that before the transmission of a control packet, it is checked whether there is enough time to send the packet before the reception of the next call packet. The received call packet can thus be forwarded immediately after it has been received.
The advantage of the method and network node of the invention is that the same transmission channel can be used for the transmission of both call packets and control packets that contain signalling information without that the quality of real-time speech transmission is impaired, since the call packets always have access to the transmission channel without delay. The transmission capacity can thus be used efficiently and no extra delay of a varying length occurs in the call transmission because of signalling internal to the system.
In a preferred embodiment of the invention the control packets are transmitted in the order of priority. The advantage is that the important control packets can be delivered as quickly as possible.
Further, in a preferred embodiment of the invention that can be used when there are several outgoing channels, the control packets are not transmitted on a certain channel, but an outgoing channel on which there is enough time to send the packet is found for each control packet. The control packets can thus be transmitted as soon as possible.
The preferred embodiments of the method and network node of the invention appear from the attached dependent claims 2 to 5 and 7 to 9.